1. Field of the Invention
The present invention relates to a hydrogen storage alloy electrode and to a sealed-type nickel-metal hydride storage battery using the same electrode.
2. Description of the Related Art
A nickel-cadmium storage battery is well known as one of sealed-type alkaline storage batteries which has recently been utilized in a variety of applications. Recently, as a battery system having a higher energy density than batteries currently available, the nickel-metal hydride storage batteries are being developed by utilizing a negative electrode comprising a metal hydride, i.e., a hydrogen storage alloy, capable of absorbing or desorbing hydrogen in a reversible manner at a low pressure. In order to realize such a hydrogen storage alloy electrode as the negative electrode of the nickel-metal hydride storage battery, the following processes have been proposed so far.
(1) A process of producing the electrode by sintering a powder of the hydrogen storage alloy together with a powder of an electrically conductive agent.
(2) A process of producing the electrode by filling or loading a powder of the hydrogen storage alloy in a three-dimensional porous metal substrate such as foamed nickel.
(3) A process of producing the electrode by causing a two-dimensional conductive support such as a punched or perforated sheet of conductor or metal to hold a mixture comprising a powder of the hydrogen storage alloy and a high polymer binder such as polytetrafluoroethylene.
Incidentally, a sealed-type alkaline storage battery configured with a hydrogen storage alloy electrode is usually designed so that the capacity of the negative electrode is larger than the capacity of the positive electrode as in the design of the nickel-cadmium storage battery. This design enables the battery to lower the inner pressure of the battery vessel, by consuming oxygen gas generated from the positive electrode during the overcharging process at the negative electrode. In addition to the generation of the oxygen gas from the positive electrode as in the case of the nickel-cadmium storage battery, the alkaline storage battery configured with the hydrogen storage alloy electrode further suffers an excessive rise in the inner pressure caused by an accumulation of hydrogen gas generated from the negative electrode.
In order to cope with this difficulty, there has been proposed a method of providing a water-repellent layer on the surface of the electrode and another method of providing a part demonstrating a hydrophilic property inside the electrode.
The negative electrode, having a two-dimensional conductive support such as a punched or perforated metal sheet as described in the above-mentioned (3), is usually rolled-up together with the positive electrode spaced by a separator in a spiral arrangement. During this rolling-up process, if the distance along a straight line linking centers of the adjacent apertures in the punched or perforated metal sheet is long, the electrode would be bent polygonally instead of circularly, thereby causing the electrode to be liable to produce cracks with burrs. If the cracks are once produced, sharp edges or burrs project into or pierce through the separator, resulting in a local short-circuiting. In this manner, a defective battery attributable to leakage current between the positive and negative electrodes may be produced.
In order to cope with this defect, a method has been proposed such that the punched or perforated metal sheet is so prepared as to have a regular perforation pattern wherein any three centers of the adjacent apertures constitute an equilateral triangle and that the electrode is rolled-up in a direction parallel to one side of any equilateral triangle.
The sintering process as described in the abovementioned (1) has a disadvantage that the surface of the hydrogen storage alloy may be oxidized to a passive state during the sintering step, and the conductivity of the electrode is lowered, thereby inviting a lowering of discharge voltage.
In addition to the inherent expensiveness of the three-dimensional porous metal substrate, the process as described in the above-mentioned (2) has a disadvantage that the produced electrode contains a portion which does not contribute to the electrode capacity in a large proportion and thus the process cannot produce an electrode of sufficient electrode capacity for the space occupied by the electrode.
The process as described in the above-mentioned (3) has a disadvantage that the process requires an addition of the high polymer binder in large quantity for causing the punched or perforated metal sheet to hold the hydrogen storage alloy powder with a sufficient bonding strength. If the large quantity of the binder is added, the conductivity of the electrode would however be lowered accordingly, thereby inviting a lowering of discharge voltage. Another disadvantage is that the electrode capacity cannot be made sufficiently large.
Further, although the above-mentioned method of providing the water-repellent layer on the surface of the electrode or providing the part demonstrating the hydrophilic property inside the electrode contributes to the lowering of the inner pressure of the battery enclosure, it has a disadvantage that it cannot satisfactorily cope with such an application as to include a rapid charging of, for instance, one hour rate or less.
Moreover, although the above-mentioned configuration of the punched or perforated metal sheet can improve the bent state of the electrode to approximate a true circle to a certain extent, it cannot however reduce fraction defects of the electrode attributable to the short-circulating a great deal.